This invention relates to the field of separation processes. More particularly, this invention relates to vapor distribution in packed distillation and absorption towers.
The purpose of a packed bed column is to provide contact area between the ascending vapor stream and the descending liquid stream. Thus packed bed columns are widely used for absorption and distillation. In an absorption tower, for example, vapor is fed to the bottom of the tower while liquid is fed to the top. As the liquid falls through the packing, it contacts the vapor rising from the bottom of the tower.
Vapor and liquid charge rates may be varied over a range of values based on tower diameter, height of the packed section, and type of packing material. Localized high vapor velocities disturb the even distribution of the falling liquid on the tower packing material and at relatively low liquid feed rates cause excessive liquid entrainment. Thus the maldistribution of vapor must be avoided to ensure optimum tower operation. Uniform vapor and liquid distribution allows maximum throughput for a given tower design. On the other hand, introducing vapor non-uniformly to a packed tower for a separation process, heat transfer, or any other heat and mass transfer operation will lower the maximum obtainable capacity and efficiency. Therefore it is important to maximize the uniformity of vapor distribution to the packed tower.
The most commonly used vapor distributor is the perforated pipe design. This device consists of a pipe or interconnected pipes with holes in the pipe walls facing downward or perpendicular to the length of the tower. Vapor flows into the empty space at the bottom of the packed tower and then flows upward into the packed bed.
The perforated-pipe vapor distributor and other previous designs have several disadvantages. First, they require substantial clearance inside the tower to provide the dead space needed for vapor distribution. Second, a decrease in the pressure drop available across the perforated-pipe distributor will cause the effluent vapor to become unevenly distributed among the perforations. Third, the high vapor velocity generated at the perforations frequently results in rapid erosion of the distributor piping. Fourth, the high pressure drop required makes the perforated-pipe type distributor undesirable for vacuum-type operations. Due to the substantial limitations of the available vapor distributors, tower performance is often compromised in the interest of operating flexibility by using a vapor inlet pipe with no distributor attached.
Fluid distribution devices designed to be attached to the outlet of conduit means are used in a broad spectrum of applications. The following is an overview of these applications.
U.S. Pat. No. 2,606,074 issued to Ackerman discloses an air distributing nozzle having radially extensive fins to be attached to the outlet of conduit means. The device distributes air evenly to defrost plate glass windows.
U.S. Pat. No. 2,621,722 to Abrams teaches a novel gas burner nozzle which distributes gas radially outward through slotted openings formed between adjacent plates in a series of parallel annular plates.
U.S. Pat. No. 3,479,146 teaches a fluid flow distributor including a dished lower baffle, a conical section intermediate baffle upstream of the dished lower baffle, and an upper annular baffle upstream of the intermediate baffle. The distributor is said to provide a uniform velocity gradient over a large range of inlet flow velocities.
A diffuser adapted to aerate a fluid medium is disclosed in U.S. Pat. No. 3,997,634 to Downs. The diffuser consists essentially of two elements, namely, a T-shaped member, the leg of which is provided with a central bore and adapted to be connected to a source of a fluid medium, such as air, under pressure, while the top is preferably flat and disc shaped. Overlying the top is a cap or cover which is flexible and adapted to lie contiguous with the surface of the top. The cap is provided with an annular rim which engages the top along and under the peripheral edge thereof. The diffuser assembly also includes a check valve to prevent back flow when the diffuser is submerged in a liquid medium.
Finally, U.S. Pat. No. 4,142,456 issued to Locker discloses an air diffuser for a conditioned air outlet. The diffuser separates the flow of air from the outlet to the ambient into a plurality of diverging air jet streams by means of a plurality of regularly disposed wedge-shaped fins, with the result that air flowing from the outlet is diffused and substantially uniformly distributed throughout an enclosure.